Even without using third party software, iOS has a surprising amount of controls that might just do the trick if you’re trying to save money. Under Settings > General > Restrictions, you can place controls on almost every aspect of the device.
You can allow/disallow Safari, FaceTime, Camera, and even Siri (useful to avoid hearing your children yell at their device all day).
Not only can you control if your children can download media or apps, but you can prevent them from uninstalling apps period (I’m sure kids would much rather have Netflix than educational apps). If you do allow them to download media and apps, you can restrict which content they can download depending on the ratings.
If you limit your kids to just Safari, you can limit which websites they go to. You can either allow all websites, restrict adult content, or only allow them to go to specific websites. Speaking from experience, the last option is CRUCIAL if you have younger children. Even if you limit adult content, certain pictures or websites do slip through the cracks.
One useful tool to monitor the apps your child download is to simply require them to ask. The Family Sharing feature allows you to force your children to require permission to download paid and free content. One caveat though, this only applies to new content, not ones they’ve previously downloaded on their device.